To manually enable use M-x set-input-method or C-x RET C-\ with haskell-unicode. Note that the elisp file must be evaluated for this to work.

Now you can simply type -> and it is immediately replaced with →. Use C-\ to toggle the input method. To see a table of all key sequences use M-x describe-input-method haskell-unicode. A sequence like <= is ambiguous and can mean either ⇐ or ≤. Typing it presents you with a choice. Type 1 or 2 to select an option or keep typing to use the default option.

If you don't like the highlighting of partially matching tokens you can turn it off:

(setq input-method-highlight-flag nil)

Abbrev mode

The Abbrev mode is not suitable since it only deals with words, not operators.